The present invention relates to a process for producing polyacrylic acids and copolymers of acrylic acid with up to 50 mol-% of comonomers. These compounds have a molecular weight of less than 100,000, measured at a pH-value of 8, and 10 to 60 mol-% are neutralized with calcium hydroxide. The present invention further relates to the use of such products as grinding and dispersing aids for calcium carbonate.
Polyacrylic acids may be applied in many fields. They may be used, e.g., as anti-deposition agent in water cycles and as grinding and/or dispersing aid in the production of pigment suspensions. In this connection the grinding and dispersion of calcium carbonate is of particular interest. Finely ground calcium carbonate is used as filler in the paper industry.
Polyacrylic acids are manufactured by polymerizing acrylic acid. In general, the polymerization is carried out in a solvent, preferably water or mixtures of water and isopropanol are used.
German Patent Application No. 34 32 082 describes the production of alkaline-earth acrylate from acrylic acid and alkaline-earth carbonate. Alkali acrylate is obtained by further reaction with alkali bicarbonate.
European Patent No. 0100948 relates to the use of polymers of acrylic acid as dispersing aids. For this purpose the polyacrylic acid is neutralized only partially.
European Patent No. 0046573 mentions that the polyacrylic acids claimed as grinding aids may be neutralized with the hydroxides of sodium, potassium, ammonium, zinc or calcium, or with primary, secondary, or tertiary amines.
According to European Patent No. 0127388 a phase separation, which can be used to purify the polymer, may be achieved by adding C.sub.1 to C.sub.5 alcohols to the neutralized aqueous polyacrylic acid solution. Sodium, potassium, lithium and ammonium are mentioned as cations suitable for the neutralization.
European Patent No. 0129329 deals with the same problem and the same cations.
In most cases the polyacrylic acid must be present in neutralized form. Sodium, ammonium, potassium, calcium, magnesium, amines or combinations thereof are usually employed as couterions. The maximum proportion of ions which--when used alone--form an insoluble polyacrylate may naturally only be so high that the polyacrylic acid is not yet precipitated. For this reason the maximum possible amount of calcium ions, for example, is 60 mol-%.
According to the teaching of European Patent No. 0100947 it is appropriate to use polyacrylic acids which are partially neutralized with calcium as grinding aids. The polymerization takes place in mixtures of isopropanol and water. On the other hand, acrylic acid partially neutralized with calcium cannot be polymerized in the presence of isopropanol without causing the precipitation of the polymer. The neutralization with calcium may only be carried out when the isopropanol has been distilled off the polyacrylic acid solution, since, otherwise, precipitation of the polymers occurs. Neutralizing the polyacrylic acid with calcium hydroxide after the polymerization has taken place involves numerous serious disadvantages. For example, it takes a long time until the calcium hydroxide dissolves in the polymer solution, in addition, the polymer solution is extremely turbid even after the neutralization.